It’s hard not to wonder if this frustratingly flat adaptation of the Edgar Rice Burroughs science-fantasy novel A Princess of Mars may have fared better under Robert Rodriguez (Machete, Sin City, Spy Kids). Of all the potential directors attached to the project over the past few decades, Rodriguez may have been the one with the most unapologetic pop and pulp sensibilities, someone who could have wallowed in the hundred-year-old adventure’s bygone excesses without feeling a need to retrofit its dated details.

Former Confederate soldier John Carter (the attractive but not especially macho Taylor Kitsch from TV’s Friday Night Lights) is prospecting out west after the war. The US army is willing to keep him behind bars until he agrees to serve as their scout. Carter escapes, his pursuers get in a standoff with American Indians and a soldier fires first. Carter rescues the wounded Col Powell (Bryan Cranston) and rides off.

That introduction to the character makes it obvious from the outset that this isn’t your great-great-grandfather’s John Carter. In the less politically correct novel, Powell is Carter’s prospecting partner ambushed by Apaches while riding alone. Carter bravely rides into the Apache camp to save Powell’s dead body from mutilation by the “savages.”

The movie’s portrayal of Dejah Thoris, the princess Carter meets after being transported to Mars, is a more significantly wrongheaded revamp. The book’s decorative and desirable Dejah is a proudly arrogant but frequently kidnapped damsel in distress. In the movie, she is a warrior, scientist and astronomer. She’s a helpless sex-object captive of the four-armed green-skinned Tharks when she first appears in the book, but in the movie she is preparing to demonstrate a new high-tech weapon of her own invention. She and her fellow “Red Martians” have copper-colored skin in the novels. The movie gives her garish facial and body tattoos that make her look like an Aztec biker chick. (Her people also wear nothing but jewelry in the books, but that obviously wasn’t an option in a Disney flick.)

Worst of all, Collins as Dejah — who is supposed to be an irresistible feminine-ideal beauty — simply isn’t very sexy onscreen. The character has been a first-tier fantasy-fiction pin-up for decades, from pulp-magazine illustrations to Frank Frazetta paperback covers to present-day “good girl art” comics. Collins looks and acts more tough and businesslike than titillating and bewitching.

The screenplay wastes too much time on Phantom Menace-murky political intrigues, putting god-like puppetmasters called Therns behind the scenes manipulating the proceedings. This storytelling blunder completely diminishes Carter’s existential adventure-story heroics, which include superheroic high-flying leaps, swordfighting and an arena confrontation with giant apes.

The Therns can morph into anyone, clothes and all, a la Mystique of the X-Men. Somehow, even in a movie about a man transported to Mars and fighting monsters, this seems ridiculously silly. (Therns are not even mentioned in the first John Carter novel, and don’t have that ability when they appear in later books.)

The computer-generated creatures are adequate but unexceptional. Also, the book’s cliffhanger ending is missing, because the film includes nothing about Mars’ atmosphere factory.

This half-hearted John Carter neither captures the book’s earnest artlessness nor goes far enough in the opposite direction to make its otherworldliness more outrageous. It’s not a complete loss, but whatever makes movies like this magical is missing.